The present invention relates to a bistable, reflective, liquid crystal display cell, having two, the compartments being separated from each other by a thin membrane, the cell being suitable for use in flat panel displays.
Conventional single cell bistable reflective display technologies are limited to a maximum reflectivity of 35% over a limited wavelength range. U.S. Pat. No. 5,384,067 and U.S. Pat. No. 5,437,811 disclose examples of such bistable reflective displays. The maximum reflectivity is further limited to 25 to 30% if the display is required to operate over a wide enough wavelength range to be white. Additionally, such displays require very high switching voltages of around 80 volts. U.S. Pat. No. 5,453,863 discloses such a white reflective display.
The contrast and/or reflection performance of displays can be improved by using a stacked structure of two or more cells. These are formed by stacking separate cells, or by making more complex cells in which an intermediate substrate is shared by both cells, forming the rear surface of the front cell and the front surface of the rear cell.
Published German Patent application No. 19711827.5, corresponding to U.S. Pat. No. 5,801,796 discloses a cell construction which enables stacked cells to be formed in which the intermediate substrate is so thin that little or no parallax occurs between the images formed in the two or more cells. In addition, the thickness of the intermediate substrate is such that the field drop across it is sufficiently low (about 25%) compared to that across the liquid crystal layers, that the stacked cell can be addressed as a single cell. This greatly simplifies and reduces the cost of the interconnections and drive electronics. The ability, with two or more cells, to absorb essentially all polarisation states of incident light and the avoidance of the need for transparent electrodes on both surfaces of the intermediate substrate also improves transmittance (and reflectivity) of the device to greater than 60%.
GB Patent application no. 9802287.4 discloses a display cell that uses two or more compartments separated by dielectric membranes. The electro-optic characteristics of the mixture in the two compartments differs in that the mixture in one of the compartments switches at a low voltage, being switched completely below the switching threshold of material in the other of the compartments. This results in contrast ratios similar to that available from cells which use polarisers, as well as levels of reflectivity similar to that available from cells that do not use polarisers, that is, as high as 80%.
Ultra-portable products increasingly require displays with almost zero power consumption, but with high pixel density, contrast and reflectivity. Such displays also have to be the minimum cost that is possible. The displays referenced above, disclosed in German patent application no. 19711827.5 and GB Patent application no. 9802287.4 both achieve high pixel density, contrast and reflectivity, but do not achieve low power and cost since they need to be continuously refreshed and require an active matrix.
So it would be desirable to provide a reflective display which has zero power consumption when the image is not being changed. This may be achieved by the use of a bistable display. It would also be desirable to provide a reflective display having a higher reflectivity than conventional displays. It would further be desirable to provide a display which is lower cost than a conventional display. Such a reduction in cost may be achieved by a display which does not require an active matrix.
Accordingly the invention provides a bistable liquid crystal display cell comprising two compartments separated by a thin transparent membrane, the first compartment containing a first liquid crystal mixture between a first substrate and a thin transparent membrane and the second compartment containing a second liquid crystal mixture between a second substrate and the thin transparent membrane, the mixtures having a first alignment in which they are both aligned parallel to the plane of the display cell, but orthogonal to each other, and a second alignment in which they are both aligned normal to the plane of the display cell, each compartment having a grating located on a surface of the compartment.
The perpendicular alignment of the first and second liquid crystal mixtures means that a minimum of incident light is absorbed by the mixtures, which results in a higher reflectivity than conventional displays.
The grating associated with the first compartment may be located on the first substrate. Alternatively, the grating associated with the first compartment may be located on a surface of the thin transparent membrane forming the first compartment.
The grating associated with the second compartment may be located on a surface of the thin transparent membrane forming the second compartment. Alternatively, the grating associated with the second compartment may be located on the second substrate.
Preferably, the liquid crystal has positive dielectric anisotropy.
More preferably, the alignment on the grating and the plane surfaces of the cell is homeotropic and the liquid crystal layer has an untwisted structure in the continuous state.
Alternatively, the alignment on the grating surface of the cell is homeotropic, the alignment on the plane surface is homogeneous and the liquid crystal layer has a twisted structure in the continuous state.
In an embodiment the membrane separating the compartments of the cell is made from an organic polymeric dielectric material.
In another embodiment, the membrane separating the compartments of the cell is made from an inorganic dielectric material.
The perpendicular alignment of the first and second liquid crystal mixtures means that a minimum of incident light is absorbed by the mixtures, which results in a higher reflectivity than conventional displays.